Cu and Sn are both tramp elements in scrap iron. They are useful because they are elements improving the corrosion resistance of steel. However, it is known that Cu causes what is called red embrittlement, which causes cracking to occur in hot working on steel (hereinafter red embrittlement induced by Cu is also referred to as “Cu embrittlement”), and Sn encourages Cu embrittlement. Thus, when a steel material containing both Cu and Sn is manufactured, the ultimate object is to inhibit surface cracking and surface defects.
Patent Literature 1 discloses a steel material having outstanding weatherability on sea shores which contains both Cu and Sn, and a structure using the same. However, this literature does not focus on prevention of surface embrittlement of a slab at hot temperature and of surface defects in continuous casting.
Patent Literature 2 discloses hot-rolled steel containing both Cu and Sn for the manufacture without occurrence of surface defects in hot working. This literature also describes that although addition of Ni to steel containing Cu makes it possible to prevent cracking on the surface of the steel which is induced by Cu, the effect of preventing cracking that Ni has diminishes on steel containing Sn in addition to Cu. However, according to this literature, Ni is considered to be a little as resources and invite high costs, and an object is to provide hot-rolled steel of a good surface property without addition of Ni. There is no enough description of the effect when Ni coexists with Cu and Sn.
Patent Literature 3 discloses the art an object of which is to prevent surface defects from occurring through continuous casting with the ratios of the components, Cu/Sn and (Cu+Ni)/Sn of anticorrosive low-alloy steel of predetermined ranges.
Each steel of Patent Literatures 2 and 3 is low-alloy steel whose content of Sn is more than twice that of Cu. The upper limit of the value of the ratio of the components, Cu/Sn (% by mass) (hereinafter referred to as “Cu/Sn ratio”) of the steel of these Literatures is 0.5. If the Cu/Sn ratio is too high, surface cracking occurs. Thus, it is difficult to improve the Cu/Sn ratio for the purpose of improvement of a property like corrosion resistance.
Non Patent Literature 1 lists the following a and b as the influences of Cu and Sn on cracking in hot working due to red shortness (liquid embrittlement) on the surface:
a. Scales are generated on the surface of a steel material heated to 1000° C. or more because of atmospheric oxidation. In a case of steel whose content of Cu is approximately 0.3% by mass, Fe that is the main component of the parent phase is selectively oxidized, and Cu is concentrated on the surface portion of the steel material. At this time, Cu, which has a lower melting point than Fe, is separated on the surface portion of the steel metal as a liquid phase. This penetrates grain boundaries, to invite liquid membrane embrittlement.
b. Cu, Sn and Ni are all metallic elements that are more difficult to be oxidized than Fe that is the main component of the parent phase, that is, nobler than Fe. Surface cracking on a steel material is conspicuous in a case of steel containing Cu and Sn among the above elements (Cu: 0.3% by mass and Sn: 0.04% by mass) compared with steel containing only Cu among the above elements (Cu: 0.3% by mass). There occurs no surface cracking in a case of steel containing only Sn among the above elements (Sn: 0.04% by mass).
In Non Patent Literature 1, the effect of inhibiting embrittlement induced by Cu and Sn that Ni has is also examined. According to this literature, it is enough for inhibiting embrittlement of the above described steel containing only Cu to add Ni of 0.15% by mass; on the other hand, it is necessary for inhibiting embrittlement of the above described steel containing Cu and Sn to add Ni of 0.3% by mass.
As described above, Non Patent Literature 1 merely describes that Sn and Ni affect inhibition of embrittlement of the above described steel containing only Cu, and that there occurs no embrittlement to the above described steel containing only Sn.